Field of the Invention
The present invention relates in general to a color video printer, and more particularly to an optical exposure system for such a color video printer which uses conventional inexpensive liquid crystal elements of 8-gradation transmissivities and increases the number of color gradations, thus improving the image quality.
Description of the Prior Art
Conventionally, color video printers are generally provided with an optical exposure system for recording or printing an image on a photosensitive medium by producing an optical image for an electric image signal and exposing the photosensitive medium to a light of the optical image. There have been known several types of optical exposure systems for producing the optical image for the electric image signal. Recently, a fiber optics liquid crystal display (FOLCD) package, which converts the electric image signal into the optical image utilizing a liquid crystal television technique, has been developed and rapidly is becoming the generally used optical exposure system.
The known FOLCD package generally comprises a liquid crystal display (LCD) panel for producing the optical image for the electric image signal synchronously with a scanning signal, In addition, at the back and at the front of the LCD panel, respectively, them are provided a back light source for irradiating a light to the LCD panel part in order to expose the photosensitive medium to the optical image produced by the LCD panel, and a fiber optics face plate having a screen for exposing the photosensitive medium to the optical image of the LCD panel.
With reference to FIG. 1, the LCD panel of the known optical exposure system generally comprises a LCD part 1 for producing the optical image for the electric image signal by red(R), green(G) and blue(B) color lines and a color filter part 2 which adheres to the LCD part 1 in order to bring out the R, G and B colors.
As shown in FIG. 2, the equivalent circuit of the LCD part 1 is constructed by the R, G and B color lines such that three groups of liquid crystal elements (L.sub.R1 -L.sub.Rh), (L.sub.G1 -L.sub.Gh) and (L.sub.B1 -L.sub.Bh) are arranged corresponding to the R, G and B lines, each common electrode terminal of the above liquid crystal elements (L.sub.R1 -L.sub.Rh), (L.sub.G1 -L.sub.Gh) and (L.sub.B1 -L.sub.Bh) is grounded, and the signal electrode terminals of the liquid crystal elements (L.sub.R1 -L.sub.Rh), (L.sub.G1 -L.sub.Gh) and (L.sub.B1 -L.sub.Bh) are connected to the source terminals of three groups of thin film transistors (T.sub.R1 -T.sub.Rh), (T.sub.G1 -T.sub.Gh) and (T.sub.B1 -T.sub.Bh), respectively. Additionally, the gates of the thin film transistors (T.sub.R1 -T.sub.Rh), (T.sub.G1 -T.sub.Gh) and (T.sub.b1 -T.sub.Bh) are connected to the R, G and B gate lines G.sub.R, G.sub.G, and G.sub.B by the R, G and B lines, respectively. The drain terminals of the thin film transistors (T.sub.R1, T.sub.G1 and T.sub.B1), (T.sub.R2, T.sub.G2 and T.sub.B2), - - - (T.sub.Rh, T.sub.Gh and T.sub.Bh) are connected, by pixels, to the drain lines D.sub.1, D.sub.2, - - - D.sub.h, respectively.
On the other hand, the color filter part 2 comprises R, G and B color filters corresponding to the three groups of liquid crystal elements (L.sub.R1 -L.sub.Rh), (L.sub.G1 -L.sub.Gh) and (L.sub.B1 -L.sub.Bh), respectively.
The operation of the FOLCD package having the above construction will be described as follows.
If the horizontal scanning signals are applied to the R, G and B gate lines G.sub.R, G.sub.G, and G.sub.B at predetermined periods and then the signal voltages corresponding to the pixels are sequentially applied to the drain lines D.sub.1, D.sub.2, - - - , D.sub.h synchronously with the horizontal scanning signals applied to the R, G and B gate lines, the thin film transistors (T.sub.R1, T.sub.G1 and T.sub.B1), (T.sub.R2, T.sub.G2 and T.sub.B2), - - - (T.sub.Rh, T.sub.Gh and T.sub.Bh) are turned on, thereby applying the signal voltages corresponding to the pixels to the liquid crystal elements (L.sub.R1 -L.sub.Rh), (L.sub.G1 -L.sub.Gh) and (L.sub.B1 -L.sub.Bh), respectively. In accordance, the liquid crystal elements (L.sub.R1 -L.sub.Rh), (L.sub.G1 -L.sub.Gh) and (L.sub.B1 -L.sub.Bh) maintain the respective signal voltages, having been applied thereto, for one period by means of a condenser. Therefore, the transmissivities of the liquid crystal elements (L.sub.R1 -L.sub.Rh), (L.sub.G1 -L.sub.Gh) and (L.sub.B1 -L.sub.Bh) are varied in proportion to the signal voltages applied thereto.
At this time, the back light source irradiates the light to the LCD panel. In result, the LCD panel makes the quantity of light which is transmitted therethrough to be varied in proportion to the signal voltages due to the respective transmissivities of the liquid crystal elements (L.sub.R1 -L.sub.Rh), (L.sub.G1 -L.sub.Gh) and (L.sub.B1 -L.sub.Bh). The transmitted light is then applied to the color filter pan 2 in order to cause the R, G and B colon to be brought out thereby. Thereafter, the R, G and B color lights from the color filter pan 2 are transmitted through the fiber optics face plate, thereby causing the photosensitive medium to be exposed thereto.
As a result, it is possible to expose the photosensitive medium to the R, G and B color lights by using the fiber optics face plate in order to cause the color image to be transmitted onto the photosensitive medium and to print the color image of the photosensitive medium. Here, in case of using a silver dry-paper-type photosensitive medium, the photosensitive medium is exposed to the optical color image at the optical exposure system, then heated itself by a heat fixing system in order to fix the color image thereof. However, in case of using a cycolor type photosensitive medium such as a cycolor film, the photosensitive medium is exposed to the optical color image at the optical exposure system, then compressed with a receiver paper by a developing system in order to develop the color image thereof onto a receiver paper which is then heated by the heat fixing system in order to fix the color image thereof.
In the above-mentioned conventional optical exposure system for a color video printer, the number of color gradations of the printed image is substantially influenced by the number of gradations of transmissivities of the liquid crystal elements. Recently, the number of gradations of transmissivities of the liquid crystal elements have gradually increased as the technique for manufacturing the liquid crystal elements have advanced. However, the increase in the number of gradations of transmissivities of the liquid crystal elements is obliged to be limited in view of the present technical level. Additionally, the higher the number of gradations of transmissivities, the more the manufacturing process of the liquid crystal element is accompanied with difficulty, thereby causing the manufacturing cost to substantially increase. As a result, it is known that the liquid crystal elements having 8-gradation transmissivities are appropriate in view of the present level of the technique for manufacturing the liquid crystal element. Thus, 8-gradation level transmissivities are widely used. Accordingly, the known optical exposure system for the color video printer generally comprises the liquid crystal elements of 8-gradation transmissivities and thus the transmissivity of the color filter pan is obliged to be fixed at a predetermined level.
It is known that the human eyes can recognize the colors of the printed image as the natural colors without a burden only in case of a color image of at least 64 color gradations. As a result, a personal color video printer conventionally is required to have at least 64 color gradations, and a high-class color video printer is required to have at least 256 color gradations.
The known optical exposure system for color video printers provided with the conventional liquid crystal elements of 8-gradation transmissivities can vary the transmissivities of the liquid crystal elements by 8 gradations for each color R, G, B. However, the number of gradations resulting from the variance of the transmissivities of the liquid crystal elements of this system is substantially deficient in providing the desired natural color, thereby causing the printer to provide a bad image quality. In an effort to overcome the above-mentioned bad image quality owing to the limited gradations, the optical exposure system may be provided with the liquid crystal elements having an increased number of gradations of transmissivities, but this type of optical exposure system has a disadvantage of requiring a difficult manufacturing process and also an expensive manufacturing cost. In addition, them has been proposed another type of optical exposure system having a multi-layer type LCD panel resulting from arranging a plurality of liquid crystal elements of 8-gradation transmissivities on at least 2 layers so as to increase the number of gradations of transmissivities. However, this type of optical exposure system has disadvantages because of the difficulty in matching the signal voltages to the quantity of optical exposure light and also because of a complex construction.